GB2326465A

GB2326465A - A refrigeration cycle utilising a multi-component refrigerant

Info

Publication number: GB2326465A
Application number: GB9712302A
Authority: GB
Inventors: Wieslaw Henryk Isalski; John Terence Lavin; David Thomas Linnett; Philip Boustead; Adrian Joseph Finn; Terence Ronald Tomlinson
Original assignee: BOC Group Ltd; Costain Oil Gas and Process Ltd
Current assignee: BOC Group Ltd; Costain Oil Gas and Process Ltd
Priority date: 1997-06-12
Filing date: 1997-06-12
Publication date: 1998-12-23
Anticipated expiration: 2017-06-12
Also published as: GB9712302D0; GB2326465B

Abstract

A refrigeration cycle utilizing a multi-component refrigerant and including at least one step of partially condensing compressed vapour in a vapour/liquid separator (62), forming a condensate stream by separating condensate so formed from uncondensed vapour (line 68) and thereafter expanding said condensate stream through valves (106, 108) and injecting said expanded condensate stream into returning low pressure refrigerant (line 40), characterised in that said expanded condensate stream is injected into said returning low pressure refrigerant in the form of at least two sub-streams (102, 104) formed by division of said condensate, at least two of said sub-streams being injected into the returning low pressure refrigerant (40) at different temperatures of the returning low pressure refrigerant. In use of the refrigeration cycle for liquefying natural gas, the mixed refrigerant preferably comprises a mixture of nitrogen and C 1 to C 5 hydrocarbons.

Description

REFRIGERATION CYCLE USING A MIXED REFRIGERANT This invention relates to refrigeration cycles using a mixed refrigerant.

In a refrigeration cycle, low pressure vapour is compressed and the compressed vapour is thereafter cooled and condensed and the high pressure condensed stream expanded back to the low pressure to form a returning low pressure refrigerant stream which is vaporised to re-form the low pressure vapour stream for return to the compressor. The final cooling and condensation of the compressed vapour is effected in indirect counter-current heat exchange with the vaporising low pressure stream. Cooling of the material to be refrigerated is by heat exchange with the vaporising low pressure stream.

In a refrigeration cycle utilizing a multi-component refrigerant, sometimes known as a mixed refrigerant refrigeration cycle, the refrigerant stream is made up of a plurality of components having differing boiling points. The compressed vapour thus condenses over a range of temperatures and likewise the condensed refrigerant boils over a range of temperatures.

These mixed refrigerant refrigeration cycles are used extensively, especially but not exclusively for natural gas. Because of the wide use of these systems, improvements in efficiency are always being sought, both in the sense of economy of operation and in simplification of the plant. Because of the scale of the plants, especially for the liquefaction of natural gas, even small improvements can substantially affect the viability of a plant.

This invention provides an improvement to the mixed refrigerant refrigeration cycles currently in use.

According to the present invention, there is provided a refrigeration cycle utilizing a multi-component refrigerant and including at least one step of partially condensing compressed vapour, forming a condensate stream by separating condensate so formed from uncondensed vapour and thereafter expanding said condensate stream and injecting said expanded condensate stream into returning low pressure refrigerant, characterised in that said expanded condensate stream is injected into said returning low pressure refrigerant in the form of at least two sub-streams formed by division of said condensate, at least two of said sub-streams being injected into the returning low pressure refrigerant at different temperatures of the returning low pressure refrigerant.

Dividing the condensate into at least two sub-streams which are separately injected into the returning low pressure refrigerant at two or more different temperatures of the returning stream enables a progressive change in the evaporation characteristics of the low pressure refrigerant to better match the combined cooling curve of the high pressure refrigerant and the feed streams. Moreover greater operating flexibility is made available to handle variations in the composition, temperature and pressure of the gas stream to be refrigerated by the cycle and to handle variations in ambient conditions.

The condensate may be expanded before division, however, in a preferred embodiment of the invention, said at least two sub-streams are separately expanded after being formed by division of the condensate stream.

In a preferred embodiment, at least one and preferably more than one, of the sub-streams is or are sub-cooled prior to expansion. In one aspect of this embodiment, two or more of the sub-streams from a condensate stream may be sub-cooled to different temperatures prior to expansion or they may be sub-cooled to the same temperature prior to expansion.

Alternatively, the condensate stream may be sub-cooled prior to division to form the substreams. Where the substreams are sub-cooled after formation by division of the condensate stream, the degree of sub-cooling of the separate sub-streams may be optimised to minimise the amount of flash on expansion into the returning low pressure refrigerant and thus reduce the complexity and cost of equipment required to achieve good two phase distribution. Sub-cooling the sub-streams to the same temperature prior to expansion or sub-cooling the condensate stream prior to division allows the possibility of directing any flash to one location only, thus reducing the complexity of equipment required for good two phase distribution.

In another embodiment, compression of the low pressure refrigerant stream is effected in at least two stages and a condensate stream which is subsequently divided into at least two sub-streams may be obtained by cooling and partial condensation of the vapour recovered from a stage of compression other than the last stage. In other words, where compression is effected in a plurality of stages with inter-stage cooling resulting in partial condensation of the refrigerant stream, condensate thereby obtained may provide a stream which is divided into two sub-streams in accordance with the invention.

In another embodiment, at least one stream which is divided into two sub-streams in accordance with the invention may be derived from partial condensation of vapour after it has been subjected to the final stage of compression. This final stage may, in one aspect of this embodiment, be the sole stage of compression. Such condensate may be obtained, for example, from a compressor after-cooler and/or from one or more further stages of partial condensation as a result of the subsequent further cooling of compressed refrigerant vapour in indirect counter current heat exchange with returning vaporising low pressure refrigerant.

Condensate streams which are divided into sub-streams in accordance with the invention may be obtained both from inter-stage cooling and from cooling and partial condensation of the vapour after it has been subjected to the final stage of compression.

In yet another embodiment of the invention, compression of the low pressure refrigerant stream is effected in at least two stages with cooling, partial condensation and separation of the condensate so formed from uncondensed vapour after each of at least two of the stages thereby providing two or more condensate streams of different compositions and at different pressures and at least two of said condensate streams are expanded and injected into returning low pressure refrigerant at different temperatures of said low pressure refrigerant. As will be explained more fully below with reference to Figure 5, this in effect creates a complete additional refrigerant stage which in turn leads to lower power consumption and/or a reduction in heat exchanger size and also further improves the operational flexibility to handle variations in the composition, temperature and pressure of the gas to be refrigerated and to handle charges in ambient conditions. If desired one or more of these condensate streams, for example a condensate stream obtained from a compressor after-cooler and a condensate obtained from a compressor inter-stage cooler, may be divided into two or more sub-streams in accordance with the invention.

A refrigeration cycle utilizing a multi-component refrigerant wherein the compression of low pressure refrigerant vapour is effected in at least two stages of compression with cooling, partial condensation and separation from the uncondensed vapour of the condensate formed each after at least two of the said stages thereby providing two or more condensate streams of different compositions and at different pressures and wherein at least two of said condensate streams are expanded and injected into the returning low pressure refrigerant at different temperatures of said low pressure refrigerant, is the subject of our copending application No. --------------- filed on the same date as this application under our internal reference 19859.

In a still further embodiment of the invention, a refluxing exchanger may be employed in the generation and separation of condensate from uncondensed vapour in at least one step where compressed vapour is cooled and partially condensed and the condensate is separated from uncondensed vapour. A refrigeration cycle utilizing a multi-component refrigerant and including at least one step of cooling and partially condensing compressed vapour and separating condensate so formed from uncondensed vapour to form a condensate stream which is thereafter expanded and injected into returning low pressure refrigerant, and wherein a refluxing exchanger is employed in at least one of said steps of cooling and partially condensing to effect at least a part of the cooling and to separate condensate from uncondensed vapour, is the subject of our corresponding application No.

--------------------- filed on the same day as this application and under our internal reference 19861.

The invention will now be described in greater detail with reference to preferred embodiments and with the aid of the accompanying drawings in which: Figure 1 is a flow sheet of a known mixed refrigerant refrigeration cycle for use in the liquefaction of natural gas; Figure 2 is a flow sheet of the refrigeration cycle of Figure 1 modified in a first manner in accordance with the invention; Figure 3 is a flow sheet of the refrigeration cycle of Figure 1 modified in a second manner in accordance with the invention; Figure 4 is a flow sheet of the refrigeration cycle of Figure 1 modified in a third manner in accordance with the invention; Figure 5, 6 and 7 are flow sheets of further variants of refrigeration cycles in accordance with the invention; and Figure 8 is a flow sheet of another embodiment of a refrigeration cycle according to the present invention wherein a refluxing exchanger is employed in the generation and separation of a condensate stream.

In the following description, the invention will be described with reference to the liquefaction of natural gas; however, it is to be understood that the use of the -refrigeration cycles of this invention is not so limited and that they are also suitable for use, for example, in other applications, eg. for other gas liquefaction processes or for purification by partial condensation techniques.

Referring now to Figure 1 of the drawings, which provides a flow sheet of a known mixed refrigerant refrigeration cycle for the liquefaction of natural gas, the natural gas which is to be liquefied is supplied at an elevated pressure to a heat exchanger 4 through line 2 and the liquefied product is recovered through line 6. The details of the arrangement for recovering the liquefied product are not relevant to the invention and many variants are possible but in the embodiment illustrated the gas is first cooled and partially condensed to recover a heavy hydrocarbon fraction.

The condensate is separated from uncondensed gas in liquid/vapour separator 8.

Condensate is recovered in line 10 and the uncondensed gas is returned to a cooler section of the heat exchanger in line 12 for a further step of cooling and partial condensation with the further condensate being separated from the uncondensed gas in liquid/vapour separator 14. The uncondensed gas is again returned to the heat exchanger, this time to the cold end, in line 16 for final cooling and condensation after which it is recovered, expanded to an intermediate pressure through valve 18 and supplied to liquid/vapour separator 20 for separation of any uncondensed gas. Condensate recovered from the separator 20 in line 22 is further expanded to its final pressure in expansion valve 24 and supplied to liquid/vapour separator 26 from which the liquefied gas is recovered in line 6 as mentioned above. Uncondensed gas from separator 20 is returned via line 30 to be reheated in heat exchanger 4 and is then combined with condensed liquid in line 32 from separator 14 which has been expanded through valve 34. The combined stream is further warmed in heat exchanger 4 and then recovered therefrom in line 36. It is thereafter joined by the cold uncondensed gas from separator 26 in line 38.

The cooling and liquefaction of the natural gas is effected in heat exchanger 4 by indirect countercurrent heat exchange with a vaporising mixed refrigerant stream in line 40. For the liquefaction of natural gas, the mixed refrigerant preferably comprises a mixture of nitrogen and Cl to C5 hydrocarbons.

The low pressure vaporised stream recovered from the heat exchanger in line 40 is recycled for recompression in a two stage compressor having first and second stages 42, 44. After compression in the first stage 42, the vapour is transferred via line 46 for cooling in inter-cooler 48 and then passed via line 50 to vapour/liquid separator 52 for the separation of condensate formed by the cooling in the inter-cooler. The uncondensed vapour is recovered in line 54 and transferred to the second stage 44 of the compressor, the compressed vapour therefrom being collected in line 56 for transfer to after-cooler 58 where it is cooled and partially condensed. The partially condensed high pressure stream is recovered in line 60. Condensate formed as a result of cooling in the inter-cooler 48 is recovered from vapour/liquid separator 52 in line 64, pumped up to the same pressure as the stream in line 60 by pump 66, and combined with that stream for supply to the vapour/liquid separator 62.

Uncondensed vapour from vapour/liquid separator 62 is recovered overhead in line 68, by which it is then passed through heat exchanger 4 where the vapour is cooled and condensed in indirect countercurrent heat exchange with the vaporising refrigerant stream in line 40 and thereafter expanded through valve 70 into the low pressure line 40 to form the returning low pressure refrigerant stream. The condensate from vapour/liquid separator 62 is recovered in line 72, sub-cooled in heat exchanger 4 in indirect countercurrent heat-exchange with vaporising returning low pressure refrigerant in line 40, removed from the heat exchanger at an intermediate point, expanded through valve 74 to about the pressure of said returning low pressure refrigerant and injected into said refrigerant via line 76.

An arrangement according to the present invention will now be described with reference to Figure 2 which illustrates the application of the invention to the prior art process illustrated in Figure 1 and in which all lines and apparatus components common with those of Figure 1 are accorded the same reference numerals. In the refrigeration cycle illustrated in Figure 2, the modification of the arrangement of Figure 1 lies in the treatment of the condensate recovered from vapour/liquid separator 62. In accordance with the invention, this condensate is split into two sub-streams 102, 104 each of which is separately sub-cooled in heat exchanger 4 in indirect countercurrent heat exchange with the vaporising returning low pressure refrigerant in line 40 and thereafter separately expanded through valves 106 and 108, respectively, to about the pressure of said vaporising returning low pressure refrigerant and then separately injected via lines 110 and 112, respectively, into said refrigerant. In the embodiment illustrated, the two substreams 102, 104 are sub-cooled to different temperatures prior to expansion and are injected into the vaporising returning low pressure refrigerant stream at different temperatures of the stream. The expanded sub-cooled condensate sub-stream in line 102 is subjected to a lower degree of sub-cooling than the corresponding sub-stream in line 104 and, after expansion, is injected into the low pressure refrigerant stream at a higher temperature than the corresponding sub-stream in line 104. By injecting these substreams into the low pressure refrigerant stream at different levels of temperature of the low pressure refrigerant stream, the evaporation characteristics of the low pressure refrigerant are progressively changed to better match the combined cooling curve of the high pressure refrigerant in line 68 (and the liquefying gas streams in lines 2, 12 and 16) and thus improve the efficiency of the process. The degree of sub-cooling of each of the sub-streams is preferably chosen to minimise flash on expansion.

An alternative modification of the refrigeration cycle of Figure 1 in accordance with the invention is shown in Figure 3 where the pipelines and apparatus components which are common with those in Figure 1 are again accorded the same reference numerals. In this arrangement, as in that of Figure 2, the modification of the arrangement of Figure 1 lies in the treatment of the condensate recovered from vapour/liquid separator 62. In the arrangement of Figure 3, this condensate in line 72 is first sub-cooled in heat exchanger 4 in indirect countercurrent heat exchange with the vaporising returning refrigerant stream in line 40 and then is removed from the heat exchanger at an intermediate point before it is divided into two sub-streams 112 and 114. In accordance with the invention, each of these sub-streams is then separately expanded through valves 116 and 118, respectively, to about the pressure of the vaporising returning low pressure refrigerant and is then separately injected into said returning low pressure refrigerant via lines 120 and 122 at different levels of temperature of said low pressure refrigerant. As the liquid to form the two sub-streams is sub-cooled before division into the sub-streams, this allows the possibility of directing any flash to one location, thus reducing the complexity of equipment required for two-phase distribution.

Figure 4 of the accompanying drawings illustrates a modification of the refrigeration cycle of Figure 1 wherein the arrangement according to the invention is applied to condensate recovered from the uncondensed vapour recovered from vapour/liquid separator 62. In Figure 4, the pipelines and apparatus components that are common with the arrangement of Figure 1 are accorded the same reference numerals.

Whereas in the arrangement of Figure 1, the compressed uncondensed vapour recovered from vapour/liquid separator 62 in line 68 is passed through the whole of heat exchanger 4 where it is cooled and substantially completely condensed, in the arrangement shown in Figure 4, it is withdrawn from heat exchanger 4 at an intermediate point in line 68A and condensate which has formed in the stream is recovered by passing it to vapour/liquid separator 130 where uncondensed vapour is recovered overhead in line 68B and the condensate is recovered in line 132. The overhead vapour in line 68B is returned to heat exchanger 4 to complete condensation. In accordance with the invention, the condensate in line 132 is divided into two streams 134 and 136 each of which is then sub-cooled in heat exchanger 4 in indirect countercurrent heat exchange with vaporising returning low pressure refrigerant, expanded through valves 138 and 140, respectively, and injected via lines 142 and 144 respectively into the returning low pressure refrigerant stream at different temperature levels of said stream. If desired, the condensate in line 132 may be sub-cooled before it is divided into two sub-streams, e.g. as shown in Figure 3.

While only one condensate stream is shown as being recovered from the compressed vapour stream in line 68A as a result of cooling and partial condensation in heat exchanger 4, two or more such condensate streams may be separated, if desired, by removing the condensing vapour stream in line 68A at different intermediate points in the heat exchanger and recovering condensate therefrom. One or more of these condensate streams may be divided into sub-streams in accordance with the invention if desired.

However, it is also within the scope of this invention for one or more of such condensate streams to be expanded and injected into the returning low pressure refrigerant stream without splitting into sub-streams, provided that at least one condensate stream formed in the refrigeration cycle is divided into sub-streams which are separately expanded and injected into the low pressure refrigerant stream at different levels of temperature thereof.

A further improvement of the refrigeration cycles of Figures 2 to 4 is achievable if condensate recovered from a compressor inter-stage cooling step is combined with the vapour recovered from the next stage of compression before it is cooled and partially condensed; that is to say if, for example, the condensate from vapour/liquid separator 52 is pumped by pump 66 into the compressed vapour in line 56.

Figure 5 illustrates a modification of the refrigeration cycle illustrated in Figure 2. In Figure 5, the pipelines and apparatus components that are common with the arrangement shown in Figure 2 are accorded the same reference numerals. As in the arrangement shown in Figure 2, the condensate recovered from compressor after-cooler 62 is divided into two sub-streams 103 and 104 which are each separately sub-cooled in heat exchanger 4 in indirect countercurrent heat exchange with vaporising returning low pressure refrigerant, separately expanded in expansion valves 106 and 108, respectively, and thereafter injected via lines 110 and 112, respectively, into the returning low pressure refrigerant stream at different temperature levels of the refrigerant stream.

In the arrangement shown in Figure 5, the treatment of the condensate stream in line 64 recovered from compressor inter-cooler 52 is modified. In accordance with this embodiment, this condensate is directed through line 64A into the warm end of heat exchanger 4 where it is sub-cooled in indirect countercurrent heat exchange with the vaporising returning low pressure refrigerant stream, recovered from the heat exchanger at an intermediate point thereof, expanded to about the same pressure as the returning low pressure refrigerant stream in valve 150 and injected into said low pressure refrigerant stream via line 152. In this embodiment, in effect the heavier liquid condensed in the compressor inter-stage cooler 48 is employed as a separate refrigerant stream from the liquid condensed in after-cooler 58. This inter-stage condensate is sub-cooled separately and injected into the common return stream at a different, preferably higher, temperature level than the condensate from the after-cooler thereby in effect creating a complete additional refrigerant stage. Lower power consumption and/or a reduction in heat exchanger size is thereby obtainable due to the following: there is no longer any requirement for compressing the inter-stage condensate; the heavier components in the inter-stage condensate can be injected into the vaporising returning low pressure refrigerant at a temperature level where they can be most beneficial, thereby improving the match between the low pressure refrigerant H/T curve and the combined cooling curve of the high pressure streams in heat exchanger 4 resulting in an improvement in thermodynamic efficiency; the after-cooler condensate contains a lower proportion of heavy components and thus less total refrigerant fluid has to be processed by the heat exchanger below the point of injection of the inter-stage condensate; also, the return refrigerant below this point is lighter and therefore evaporates more easily, thus improving the heat transfer efficiency and reducing heat exchanger duty.

Figure 6 of the accompanying drawings shows a similar arrangement to that of Figure 5 but wherein instead of the condensate from vapour/liquid separator 62 being split into two sub-streams, it is the condensate from vapour/liquid separator 52 which is split into the two sub-streams in accordance with the invention. Thus, as in Figure 1, the condensate from vapour/liquid separator 62 is sub-cooled in heat exchanger 4 in indirect countercurrent heat exchange with vaporising returning low pressure refrigerant, and is thereafter withdrawn from the heat exchanger at an intermediate point thereof, expanded through expansion valve 74 and injected into the returning low pressure refrigerant stream through line 76.

The condensate recovered from vapour/liquid separator 52 in line 64A is divided, in accordance with the invention, into two sub-streams 160 and 162 each of which is separately cooled and partially condensed in heat exchanger 4 in indirect countercurrent heat exchange relationship with the vaporising returning low pressure refrigerant, separately expanded to about the pressure of the low pressure refrigerant stream through valves 164 and 166, respectively, and separately injected via lines 168 and 170 into said returning low pressure refrigerant stream at different temperature levels of said stream.

In the embodiment illustrated in Figure 7, both the condensate from vapour/liquid separator 52 and the condensate from vapour/liquid separator 62 are split into sub-streams in accordance with the invention.

In each of the embodiments illustrated in Figures 5, 6 and 7 the splitting of the condensate or condensates into sub-streams and the subsequent expansion of the substreams may be effected after the sub-cooling of the condensate, e.g. as illustrated in Figure 3.

Further improvement in the efficiency of the refrigeration cycle according to the invention is achievable if a refluxing exchanger is employed in the generation and separation of a condensate. An application of this embodiment of the invention is illustrated in Figure 8 which is a modification of the arrangement of Figure 5 and wherein all pipelines and apparatus components common with Figure 5 are accorded the same reference numerals.

In the arrangement of Figure 8, the after cooler 58 and liquid/vapour separator 62 of the arrangement of Figure 5 are replaced by a reflux exchanger 180. The compressed refrigerant recovered from the final stage 44 of compression is directed via line 56 to reflux exchanger 180 where it is cooled and partially condensed while being directed upwardly through the exchanger. Uncondensed vapour is recovered form the top of the exchanger through line 68 while condensate formed in the exchanger travels back down through the exchanger in direct countercurrent contact with the rising vapour and is collected from the bottom of the exchanger in line 72. As a result of replacing the aftercooler 58 and liquid/vapour separator 62 by the refluxing exchanger 180, the concentration of light components in the condensate in line 72 can be minimised thus enabling the sub-streams derived from the condensate to be sub-cooled in lines 102 and 104 to a temperature where little or no flash occurs on expansion into the returning low pressure refrigerant. This greatly reduces the complexity and cost of equipment necessary for achieving good two-phase distribution. There is also a concomitant reduction in the heavy hydrocarbon content of the vapour leaving the reflux condenser in line 68, thus reducing the circulating flow and improving the thermodynamic efficiency in the lower temperature sections of the refrigerant circuit.

While in the refrigeration cycle of Figure 8, a refluxing exchanger replaces the compressor after-cooler 58 and associated vapour/liquid separator 62 of the cycle of Figure 5, it will be understood that it may also be used to replace the compressor intercooler 48 and associated liquid vapour separator 52. It may likewise be employed to replace corresponding equipment in the refrigeration cycles of any of Figures 2 to 4, 6 and 7. If it is employed to replace the compressor after-cooler and associated vapour/liquid separator of the cycles of any of Figures 2 to 4, the condensate in line 64 will suitably be injected into the rising vapour in the refluxing exchanger at an intermediate point in the exchanger, the location of which is determined by the temperature and composition of the said condensate.

Where used, the refluxing exchanger may provide less than all the cooling and thus may be used in series with a conventional inter-cooler or after-cooler as well as a total replacement therefor.

Refluxing exchangers may possibly also be used in the generation and separation of other condensate streams in the refrigeration cycle where these streams are derived by partial condensation of compressed refrigerant.

Further variants will be apparent to those skilled in the art. For example, any of the refrigeration cycles illustrated in Figures 5, 6 and 7 may be further modified by recovering the high pressure stream from line 68 at one or more intermediate points of the heat exchanger 4, separating condensate therefrom, expanding the condensate to about the pressure of the returning low pressure refrigerant stream and injecting it into that stream. Any such condensate stream may, if desired, be split into sub-streams which are separately expanded and injected into the low pressure refrigerant stream in accordance with the invention.

While in all the embodiments shown in the drawings, two sub-streams are formed from a condensate stream, it will be understood that three or more sub-streams may be formed from the condensate stream if desired.

While heat exchanger 4 is shown as being a single heat exchanger, its overall functions may be supplied by a plurality of exchangers.

It will generally be preferred for at least any heat exchanger employed in the indirect counter-current heat exchange of compressed refrigerant with returning low pressure refrigerant to be a multi-stream plate fin type heat exchanger because such heat exchangers provide greater flexibility to efficiently process a multiplicity of different streams.

One or more of the expansion valves employed for the expansion of condensate in any part of the refrigeration cycle may, if desired, be replaced by devices in which expansion is effected with performance of e other streams or for purification where one or more contaminants is or are removed by cooling and partial condensation. Examples include air separation, the treatment of refinery off gas, and the liquefaction of ethylene and ethane.

Any suitable combination of two or more refrigerants may be used in the mixed refrigerant cycle and the choice will depend upon the composition of the material to be refrigerated and the temperature to which it is to be cooled. Examples of suitable refrigerants include nitrogen, low boiling halogenated hydrocarbons, eg.

chlorofluorocarbons, and low boiling hydrocarbons. In general however, the mixed refrigerant will usually comprise two or more of nitrogen and Ci-Cs hydrocarbons.

Claims

1. A refrigeration cycle utilizing a multi-component refrigerant and including at least one step of partially condensing compressed vapour, forming a condensate stream by separating condensate so formed from uncondensed vapour and thereafter expanding said condensate stream and injecting said expanded condensate stream into returning low pressure refrigerant, characterised in that said expanded condensate stream is injected into said returning low pressure refrigerant in the form of at least two sub-streams formed by division of said condensate, at least two of said sub-streams being injected into the returning low pressure refrigerant at different temperatures of the returning low pressure refrigerant.

2. A refrigeration cycle as claimed in claim 1 wherein said at least two substreams are expanded after being formed by division of the condensate.

3. A refrigerant cycle as claimed in Claim 1 or Claim 2 wherein at least one of the sub-streams is sub-cooled prior to expansion.

4. A refrigeration cycle as claimed in Claim 3 wherein at least two of the sub streams are sub-cooled to different temperatures prior to expansion.

5. A refrigeration cycle as claimed in Claim 3 wherein at least two of the sub streams from a condensate stream are sub-cooled to the same temperature prior to expansion.

6. A refrigeration cycle as claimed in Claim 5 in which the condensate stream is sub cooled prior to division.

7. A refrigeration cycle as claimed in any one of Claims 1 to 6 wherein compression of the low pressure refrigerant stream is effected in at least two stages and at least one said condensate stream which is subsequently divided into said at least two sub-streams is obtained by cooling and partial condensation of the vapour recovered from a stage of compression other than the last stage.

8. A refrigeration cycle as claimed in any one of Claims 1 to 6 wherein compression of the low pressure refrigerant stream is effected in at least two stages and at least one said condensate stream which is subsequently divided into said at least two sub-streams is obtained by cooling and partial condensation of vapour recovered from the last stage of compression.

9. A refrigeration cycle as claimed in Claim 8 in which condensate obtained from the after-cooler from the last stage of compression provides a said condensate stream which is divided into two sub-streams.

10. A refrigeration cycle as claimed in Claim 8 or Claim 9 wherein condensate and vapour streams recovered separately following partial condensation after a stage of compression other than the last stage are separately raised in pressure in the next stage of compression, the vapour stream is thereafter cooled and partially condensed and the streams are then recombined.

11. A refrigeration cycle as claimed any one of Claims 1 to 10 wherein the compressed vapour is cooled at least in part by indirect counter-current heat exchange with returning low pressure refrigerant, said cooling comprises one or more steps of cooling and partial condensation with separation of condensate formed after each step from uncondensed vapour, and a said condensate stream which is divided into at least two sub-streams is derived from a said separated condensate.

12. A refrigeration cycle as claimed in any one of Claims 1 to 11 wherein compression of the low pressure refrigerant stream is effected in at least two stages with cooling, partial condensation and separation of the condensate so formed from uncondensed vapour after each of at least two of the stages thereby providing two or more condensate streams of different compositions and at different pressures and wherein at least two of said condensate streams are expanded and injected into returning low pressure refrigerant at different temperatures of said low pressure refrigerant.

13. A refrigeration cycle as claimed in Claim 12 wherein at least one of said two or more condensate streams of different composition provides a said condensate stream which is divided into said at least two sub-streams.

14. A refrigeration cycle as claimed in Claim 12 wherein the condensate stream derived by partial condensation after the last stage of compression and a condensate stream derived by partial condensation after a previous stage of compression each provides a said condensate stream which is divided into said two sub-streams.

15. A refrigeration cycle as claimed in any one of the preceding claims in which a refluxing exchanger is employed in the generation and separation of condensate from uncondensed vapour in at least one of the separation steps.

16. A refrigeration cycle as claimed in any one of the preceding claims utilized for the liquefaction of natural gas.

17. A refrigeration cycle as claimed in any one of the preceding claims wherein the refrigerant comprises a mixture comprising any combination of two or more of nitrogen and Cl to C5 hydrocarbons.

18. A refrigeration cycle as claimed in any one of the preceding claims wherein one or more multi-stream plate fin type heat exchangers is or are employed in the cooling and partial condensation of compressed refrigerant.